1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a facsimile, a printer, etc., and more particularly to an image forming apparatus capable of preventing a lifetime of an electrostatic latent image bearing member from being decreased by friction with an appropriate supply of a lubricant to the electrostatic latent image bearing member such as a photoconductive element.
2. Discussion of the Background
In an electrophotographic image forming apparatus, such as a copying machine, a facsimile, a printer, etc., such an image forming apparatus is well known that includes an image forming section having an electrostatic latent image bearing member to form an electrostatic latent image on its surface, a charging device to uniformly charge the electrostatic latent image bearing member, a developing device to develop the electrostatic latent image formed on the surface of the electrostatic latent image bearing member into a visible image and a cleaning device to remove developer adhered to the electrostatic latent image bearing member.
FIG. 9 is a schematic drawing illustrating an exemplary construction of an image forming section 1 in an electrophotographic image forming apparatus in the art. A part having the reference numeral of 10 in FIG. 9 is a photoconductive element, for example an electrostatic latent image bearing member, and a surface of the photoconductive element 10 is uniformly charged by a charging roller 11. A desired electrostatic latent image is formed on the surface of the charged photoconductive element 10 with an exposure light 12 irradiated from an optical writing device (not shown). The electrostatic latent image is then developed into a visible toner image with toner in a developing device 13. The toner image formed on the surface of the photoconductive element 10 is transferred onto a transfer sheet carried on a transfer sheet conveying belt 14 with a transfer bias applied by a transfer brush 15. The transfer sheet having the transferred toner image is conveyed to a fixing device (not shown) where the toner image is fixed onto the transfer sheet by being heated and pressed. A part having the reference numeral of 16 is a cleaning device including a cleaning blade 17. The cleaning device 16 is provided so as to remove residual toner remaining on the surface of the photoconductive element 10 with the cleaning blade 17. The removed toner is conveyed to a used toner container (not shown) by a used toner conveying screw 19. Further, a cleaning brush roller 18, which is a cleaning support member, to roil and remove toner on the surface of the photoconductive element is disposed at an upstream side of the cleaning blade 17 in the moving direction of the photoconductive element.
An image forming apparatus illustrated in FIG. 10 includes four image forming sections. Each of which has the same construction as that illustrated in FIG. 9 and is located in parallel to form images of different colors. The image forming apparatus also includes a transfer sheet conveying belt 14 to convey a transfer sheet to the four image forming sections 1B, 1C, 1M and 1Y. Reference numerals 12Y, 12C, 12M and 12B represent exposure light irradiated in the four image forming sections 1Y, 1C, 1M and 1B respectively. In the back surface side of the transfer sheet conveying belt 14, transfer brushes 15B, 15C, 15M and 15Y, which transfer a toner image formed at each image forming section 1B, 1C, 1M and 1Y, are provided opposing to the respective photoconductive elements 10B, 10C, 10M and 10Y so as to transfer the toner image formed at each image forming section 1B, 1C, 1M and 1Y onto a transfer sheet carried on the transfer sheet conveying belt 14 one after another superimposing each color toner image on each other. In an image forming apparatus illustrated in FIG. 10, a desired full color image can be obtained when each color of toner; black (B), cyan (C), magenta (M) and yellow (Y) is used at respective developing devices 13B, 13C, 13M and 13Y of the image forming sections 1B, 1C, 1M and 1Y. Such an image forming apparatus as described above is commonly known as a tandem color image forming apparatus.
A technology is widely known which suppresses an abrasion of a surface of a photoconductive element caused by a cleaning blade, etc., by supplying the photoconductive element with a lubricant and thereby preventing a lifetime of the photoconductive element from being decreased due to the abrasion, and various systems for supplying a lubricant are proposed. Two systems are known to supply a lubricant. A first type supplies the lubricant by pressing it directly against a surface of a photoconductive element (for example, in Japanese Patent Laid-Open Publication No. 9-62163). A second type supplies the lubricant by abutting a rotatable brush roller on the photoconductive element while abutting the lubricant on the rotatable brush roller (for example, in Japanese Patent Laid-Open Publication No. 6-324603). In the latter system, since a cleaning device to remove residual toner remaining on the surface of the photoconductive element is often configured to include a cleaning brush roller, such a system has been proposed that utilizes the cleaning brush roller as the above-described brush roller.
A plurality of image forming sections in the above-described tandem image forming apparatus are configured to have the same structure as to each other. However, a supply of a lubricant to a photoconductive element has to be adjusted to a proper amount in each image forming section.
An electrical action for a lubricant in a transfer section will now be discussed. Fluorine-containing resins polytetrafluoroethylene (four fluoridation ethylene resin) and PVDF (polyvinylidene fluoride) used as a lubricant electrically have negative characteristics. In a reverse development that uses negatively charged toner, positive bias is applied to a transfer section. Consequently, a force is exerted that electrically removes the lubricant adhered to a photoconductive element.
FIG. 11 explains a change in an amount of a removed lubricant over a lapse of time according to a different voltage level of a transfer bias. In this explanation, a coefficient of friction on a photoconductive element is measured as substitutive characteristics of an amount of a lubricant on a surface of the photoconductive element. It is observed that the higher a voltage of the transfer bias to be supplied, the larger the amount of the lubricant is removed when the apparatus is operated by varying the voltage level of the transfer bias with using a photoconductive element possessing a nearly constant coefficient of friction.
In the above-described tandem color image forming apparatus having four image forming sections, a technology is commonly known in which a smooth transfer is achieved by intensifying a transfer bias to be applied to an image forming section in order of image forming section from an upstream to a downstream in a moving direction of a transfer sheet conveying direction. In the configuration that intensifies the transfer bias in order of the image forming section from the upstream to the downstream, a lubricant will be easier to be removed from a surface of a photoconductive element in order of the image forming section from the upstream to the downstream, and an application of a lubricant to increase a lifetime of a photoconductive element will become less effective.
A proper amount of a lubricant must be supplied so as not to be inconvenienced by a mixture of a foreign substance due to an accumulated lubricant on a surface of a photoconductive element. It is preferable that the lubricant on the surface of the photoconductive element is removed completely before the next supply of lubricant is made after the lubricant has been supplied so that a fresh lubricant layer is always formed on the surface of the photoconductive element.
Further, when a mechanical condition in each image forming section is to be differentiated, it is advantageous from the view point of cost effectiveness to achieve the differentiation with minimum replacement parts while maintaining a maximum commonality of parts.